miniature orchids and plants for terrarium
 

How small a container can I use for a closed environment growing chamber for miniature plants?

I don't use terrariums, so hopefully someone will chime in with more information, but I'd say a rough size of a 10 gallon aquarium would be the starting point, 20 gal. being even better. The smaller the tank the harder it is to grow Orchids well, most of them need good air movement to flourish. Again, I'm no expert on the matter.

At least 10 gallon for real miniatures, and no larger than 90 gallons would be my advice. If you go too small, you'll basically be able to stuff 2 phals in there and that's it. If you go too big, humidity can be difficult to maintain, unless you develop some sort of irrigation system/transform it into an actual terrarium type thing. I'm assuming you're interested in building just a functional grade tank for growing humid-loving species, rather than a habitat (which are amazing as well!)